5 July 2002

Are strobs cause of leaf spotting?

By Andrew Blake

STROBILURIN fungicides could be partly to blame for potentially yield-sapping leaf spots on Scottish winter barleys, according to SAC pathologist Simon Oxley.

Last year, spotting caused by ramularia fungus and/or environmental factors cut Scottish spring barley yields by 0.25t/ha and increased screenings by 2%.

Until this year the problem has rarely been troublesome on the winter crop, says Dr Oxley. "But it has come in quite early this year."

Provided spray timings are correct, most strobilurin fungicides offer some control of ramularia and can reduce physiological spotting. But their protectant effects do not last forever, he says. And the first disease to return in the absence of others may be ramularia. "You could be ending up with a weak disease that has a clean leaf to grow on."

According to Keith Dawson of CSC Crop Care, new strobs are faring better than older ones at controlling the pathogen. Recent checks show pyraclostrobin-based products (eg Opera) outperforming Twist (trifloxystrobin), which was in turn ahead of Acanto (picoxystrobin).

Widespread spotting on winter barley is a warning to get spring crops protected as soon as possible, he adds. But he dismisses suggestions that Corbel (fenpropimorph) could be contributing to spotting by dewaxing leaves. &#42

Strob fungicides may be contributing to leaf spotting, says Simon Oxley, who has found some winter barleys resist it better than others (see table).


Physiological Ramularia

Leonie 66% 48%

Siberia 49% 16%

Regina 49% 12%

Pastoral 33% 5%

Angela 31% 23%

Muscat 26% 8%

Pearl 26% 7%

Sumo 21% 6%

Antonia 19% 7%

Source: SAC.